The control of gene expression is at the core of biological development and homeostasis. In recent years, an unexpected form of gene regulation has been discovered in which small RNA molecules known as microRNAs (miRNAs) repress the expression of target genes through RNA interference. There are over 500 miRNAs in the human genome, and collectively they may regulate 20-30% of all genes. Virtually nothing is known regarding the function of miRNA in liver development and disease. To address this, we have quantified the expression of all miRNAs during mouse liver development. We have identified a set of miRNAs whose expression rises significantly during the period of hepatobiliary differentiation and morphogenesis, and we have begun to localize their expression by in situ hybridization. This approach has yielded the first examples of developmentally regulated miRNAs in the embryonic liver. One miRNA (miR-30a) is expressed in the ductal plate and bile ducts. Our preliminary work on miR-30a has resulted in the first evidence of a requirement for a miRNA in biliary development. The function of miR-30a and other hepatic miRNAs will be assessed in a cell culture model of liver differentiation (Aim 1A). This system will be also be used to identify the molecular targets of hepatic miRNAs (Aim 1A). The mouse genetic model will be used to test the developmental function of miR-30a and selected miRNAs in vivo (Aim 1B). Many diseases are caused by aberrations in normal developmental pathways. Biliary atresia, the leading indication for pediatric liver transplantation, is associated with developmental anomalies. Despite considerable efforts, its cause is unknown. The role of miRNA in biliary atresia has never been explored, although we have found that miR-30a is highly expressed in the proliferating bile ducts characteristic of biliary atresia. We will utilize specimens collected as part of the Biliary Atresia Resarch Consortium clinical study to characterize the expression of all miRNA in biliary atresia tissue and controls (Aim 2A). This will be complemented by a parallel study using a mouse model of biliary atresia. The pathogenic role of miRNA will then be tested by inhibition of miRNAs in the mouse model (Aim 2B). These studies have the potential to shed light on biliary atresia pathogenesis and may result in new treatments. The research proposed is significant because it will reveal novel regulatory pathways in normal liver development and in biliary atresia. It directly addresses goals of the NIH Action Plan for Liver Research regarding liver development and identification of the cause of biliary atresia.